I'm not 100% sure if the same applies to frillies but with bearded dragons and a lot of other lizards and geckos, that is "somewhat" true. The true part is yes you can tell the sex of a lizard by the bulges at the base of the tail, the males are usually very prominent and females don't have any bulge. The untrue part is that you cannot see this until the reptile is much older than 3 months. In most reptiles they'd have to be at least 6 months before the bulges are prominant enough to tell, Most reptiles younger than that are indistinguishable. It sounds like some of the facts in that book got lost in translation so be careful with what it tells you.